Lets
look at the same matter in a more practical way, going back to the imaginary example of
slides 5-7.

Here we have: R0 = 3, R = 1, (S/N)

eq = 1/3, and (I/N)eq = PC =
2/3.

In words: this microorganism in this population would be transmitted, on
average, to three susceptibles per each infected case should all members of the population
be susceptible (R0=3); on average, the infection is actually
transmitted, to only one susceptible person per infected case (R=1); in this state of
equilibrium, one out of three people are susceptible ((S/N)

eq = 1/3) and two out of three are immune ((I/N)eq = 2/3); if we could achieve a vaccination level higher than 2/3 we
would be able to eventually eradicate the microorganism (PC = 1 - (1/R0)
= 1 - (1/3) = 2/3 = 67%).